In the past attempts have been made to achieve, in the slice flow of the stock suspension, an even cross-directional profile of the machine direction velocity. Likewise, it has been known that detrimental cross velocities may occur in the said flow. Especially this has caused problems in the edge areas of the web, for instance, by intensifying the edge wave.
The reason for achieving even velocity is to produce paper that is homogeneous as to its grammage, formation and strength along its entire width.
A web produced with any of the present paper machine headboxes is more or less non-homogeneous in the edge areas. There are several methods to detect non-homogeneity; e.g. by measuring the characteristics of the paper or board web. Characteristics to be measured might include e.g. the grammage, tensile strength (cross direction), elongation (cross direction), tearing resistance and other similar variables. When measuring the above-mentioned variables at various points across the web, it is seen that the characteristics measured at the edges and those measured in the middle area of the web are unequal. This unequality of the characteristics indicates variations in the quality of the web and in the extreme cases portions of the web width might have to be rejected. The critical degree of the unequality in characteristics is dependent on the quality and use of the paper. To illustrate this, laser copying paper might be mentioned; the fiber orientation in this kind of paper has to be very even and, at the same time, its homogeneity very high.
The impact of the headbox slice on these variables has for instance been dealt with in the following sources:
(1) E. Weisshuhn und Dahl Einfluss des Stoffauflaufs auf Blatteigenschaften und deren Konstanz, Das Papier 10A, 1986 p. 151-164. PA1 (2) D. Egelhof Der Einfluss des Stoffauflaufes auf Asymmetriefehler im Papier, Das Papier 7, 1986 p. 313-318. PA1 (3) P. Soikkanen Sym-flow, the versatile headbox. Fifth Valmet Paper Machine Days 1986 PA1 E. Weisshuhn et al. DE patent application No. 3 514 554
Various alternatives to correct the asymmetry are presented in these sources.
In addition to these, various reasons and solution models for stabilizing the jet flow across the entire width of the web, have been presented in reference (4)
The solutions presented in reference (4) require various adjustments when the flow and production conditions change and some of the solutions are expensive to realize and their reliable function is questionable. The solution presented in reference (3) requires very complicated adjustment procedures in all production conditions. The procedure in question involving several test samples and time-consuming measurements is not to be recommended for a process which is very fast and has high operating costs.
It is also previously known to remove a small portion of the stock flow on each side of the channel between the slice and the rectifier roll of the headbox, before it is discharged onto the wire (e.g. Finnish patent No. 43 812. Beloit Corporation). Also previously known is a solution contrary to this where an additional flow of water is introduced through the side wall (Finnish patent No. 30095, Valmet Oy) but, as far as is known, the latter has not been realized in practice.
A method and apparatus for controlling fiber orientation misalignment in the paper web in the paper machine headbox is disclosed in Valmet Oy's Finnish patent application Nos. 844276 and 850638. In the method of the Finnish application No. 844276 medium flows are introduced at both opposite edge areas of the flow channel thereby controlling the fiber orientation misalignment.
In the method of the Finnish application No. 850638 the edge flows are introduced via edge flow channels fitted on both sides of a turbulence generator or the like of the headbox, and to control the fiber orientation misalignment, the volume of flows at the edges is controlled by adjusting the cross sectional area of the edge flow channels by special means.
There is an increasing need for adjusting the stock jet velocity profile in fast paper machines, when the object is to affect the fiber orientation. An even fiber orientation in the paper web is essential also because, in the drying section, when the paper shrinks, the degree of shrinkage is highest in the length direction of the fibers. With an even degree of shrinkage, a paper web of even tightness across the entire width of the we is obtained. The leaning tendency of a stack of forms has also been observed to be due to unequal fiber orientation.